1. Field of the Invention
This invention relates to radiation cross-linked conductive polymer PTC compositions and devices comprising them.
2. Introduction to the Invention
Conductive polymer compositions, and devices comprising them, have been described in published documents and in previous applications assigned to the same assignee. Reference may be made for example to U.S. Pat. Nos. 2,978,665 (Vernet et al.), 3,243,753 (Kohler), 3,351,882 (Kohler et al), 3,571,777 (Tully), 3,793,716 (Smith-Johannsen), 3,823,217 (Kampe), 3,861,029 (Smith-Johannsen), 4,017,715 (Whitney et al), 4,177,376 (Horsma et al), 4,237,441 (Van Konynenburg et al), 4,246,468 (Horsma) and 4,272,471 (Walker); U.K. Patent No. 1,534,715; the article entitled "Investigations of Current Interruption by Metal-filled Epoxy Resin" by Littlewood and Briggs in J. Phys D: Appl. Phys, Vol. II, pages 1457-1462; the article entitled "The PTC Resistor" by R. F. Blaha in Proceedings of the Electronic Components Conference, 1971; the report entitled "Solid State Bistable Power Switch Study" by H. Shulman and John Bartho (August 1968) under Contract NAS-12-647, published by the National Aeronautics and Space Administration; J. Applied Polymer Science 19, 813-815 (1975), Klason and Kubat; Polymer Engineering and Science 18, 649-653 (1978) Narkis et al; and commonly assigned U.S. Ser. Nos. 601,424 (Moyer), now abandoned, published as German OLS 2,634,999; 750,149 (Kamath et al), now abandoned, published as German OLS No. 2,755,077; 732,792 (Van Konynenburg et al), now abandoned, published as German OLS No. 2,746,602; 751,095 (Toy et al), now abandoned, published as German OLS No. 2,755,076; 798,154 (Horsma et al), now abandoned, published as German OLS No. 2,821,799; 965,344 (Middleman et al), published as German OLS No. 2,948,281 U.S. Pat. No. 4,238,812; 965,345 (Middleman et al) now abandoned, published as German OLS No. 2,949,173; 6,773 (Simon), published as German OLS No. 3,002,721 U.S. Pat. No. 4,255,698; 67,207 (Doljack et al), now abandoned, published as European Patent Application No. 26,571; 88,304 (Lutz), now abandoned, published as European Patent Application No. 28,142; 98,711 (Middleman et al) now U.S. Pat. No. 4,315,237; 141,984 (Gotcher et al) now abandoned, published as European Patent Application No. 38,718; 141,987 (Middleman et al) now U.S Pat. No. 4,413,301; 141,988 (Fouts et al) now abandoned, published as European Patent Application No. 38,713; 141,989 (Evans) now U.S. Pat. No. 5,049,850; 141,991 (Fouts et al) now U.S. Pat. No. 4,545,926; 142,053 (Middleman et al), now U.S. Pat. No. 4,352,083; 142,054 (Middleman et al), now U.S. Pat. No. 4,317,027; 150,909 (Sopory), now abandoned; 150,910 (Sopory), now U.S. Pat. No. 4,334,351; 150,911 (Sopory), now U.S. Pat. No. 4,318,881; 254,352 (Taylor), now U.S. Pat. No. 4,426,633; 300,709 (Van Konynenburg et al) now abandoned, published as European Patent Application No. 74,281; and the application filed on Feb. 17, 1982 by McTavish et al now U.S. Pat. No. 4,481,498. The disclosure of each of the patents, publications and applications referred to above is incorporated herein by reference.
Conductive polymer compositions are frequently cross-linked, e.g. by radiation, which is generally preferred, or by chemical cross-linking, in order to improve their physical and/or electrical characteristics. Compositions exhibiting PTC behavior, which are used in self-limiting heaters and circuit protection devices, are usually cross-linked to ensure that the resistivity of the composition remains at a high level as the temperature of the composition is increased above the switching temperature (T.sub.s) of the composition. The extent of cross-linking which has been used in practice has in general been relatively low; thus the dose used in radiation cross-linking has typically been 10 to 20 Megarads. Cross-linking by radiation using higher doses has, however, been suggested in the literature. Thus U.S. Pat. No. 3,351,882 (Kohler et al) discloses the preparation of a resistor comprising a melt-extruded PTC conductive polymer element and two planar electrodes embedded therein, followed by subjecting the entire resistor to about 50 to 100 megarads of radiation of one to two million electron volt electrons in order to cross-link the conductive polymer, particualrly around the electrodes. Ser. No. 601,424 (Moyer), now abandoned, published as German OLS 2,634,999, recommends radiation doses of 20 to 45 megarads to cross-link a PTC conductive polymer, thus producing a composition which has high peak resistance and maintains a high level of resistivity over an extended range of temperatures above T.sub.s. U.K. Specification No. 1,071,032 describes irradiated compositions comprising a copolymer of ethylene and a vinyl ester or an acrylate monomer and 50-400% by weight of a filler, e.g. carbon black, the radiation dose being about 2 to about 100 Mrads, preferably about 2 to about 20 Mrads, and the use of such compositions as tapes for grading the insulation on cables.